refactor: 全面集成 slog 日志到 handler、service、middleware

- cmd/main.go: 集成 logger.Init 和日志中间件，替换标准 log 包
- handler 层: 5xx 错误记录完整日志，返回通用消息（防内部信息泄露）
- service 层: LLM/图片生成/存储/认证等关键操作补充结构化日志
- auth 中间件: 记录认证失败原因
- generate.go: 后台管线任务使用带 task_id 的 logger

backend/internal/service/storage.go

@@ -9,6 +9,7 @@ import (
 	"time"
 
 	"gen2d/internal/config"
+	"gen2d/internal/logger"
 
 	"github.com/qiniu/go-sdk/v7/storagev2/credentials"
 	"github.com/qiniu/go-sdk/v7/storagev2/objects"
@@ -39,8 +40,11 @@ func NewStorageService(cfg config.QiniuConfig) *StorageService {
 
 // Upload 上传图片字节到七牛云，返回 CDN 访问 URL。
 func (s *StorageService) Upload(ctx context.Context, key string, data []byte) (string, error) {
+	l := logger.FromCtx(ctx)
+
 	putPolicy, err := uptoken.NewPutPolicy(s.bucket, time.Now().Add(1*time.Hour))
 	if err != nil {
+		l.Error("qiniu put policy failed", "key", key, "error", err)
 		return "", fmt.Errorf("qiniu put policy: %w", err)
 	}
 	upToken := uptoken.NewSigner(putPolicy, s.creds)
@@ -49,14 +53,19 @@ func (s *StorageService) Upload(ctx context.Context, key string, data []byte) (s
 		UpTokenProvider: upToken,
 	})
 
+	l.Info("uploading to qiniu", "key", key, "size", len(data))
 	err = uploadMgr.UploadReader(ctx, bytes.NewReader(data), &uploader.ObjectOptions{
 		BucketName: s.bucket,
 		ObjectName: &key,
 	}, nil)
 	if err != nil {
+		l.Error("qiniu upload failed", "key", key, "error", err)
 		return "", fmt.Errorf("qiniu upload: %w", err)
 	}
-	return s.buildURL(key), nil
+
+	cdnURL := s.buildURL(key)
+	l.Info("upload succeeded", "key", key, "url", cdnURL)
+	return cdnURL, nil
 }
 
 // GetDownloadURL 生成素材的下载 URL。
@@ -67,8 +76,17 @@ func (s *StorageService) GetDownloadURL(_ context.Context, key string) (string,
 
 // Delete 从七牛云删除指定对象。
 func (s *StorageService) Delete(ctx context.Context, key string) error {
+	l := logger.FromCtx(ctx)
+
 	bucket := s.objectsMgr.Bucket(s.bucket)
-	return bucket.Object(key).Delete().Call(ctx)
+	err := bucket.Object(key).Delete().Call(ctx)
+	if err != nil {
+		l.Error("qiniu delete failed", "key", key, "error", err)
+		return err
+	}
+
+	l.Info("delete succeeded", "key", key)
+	return nil
 }
 
 // buildURL 根据配置构建完整的 CDN URL。